The Nuclear Regulatory Commission (search) has concluded that it is unlikely significant amounts of radiation would be released in a deliberate crash of a jetliner into a nuclear power plant, but that engineering tests have not entirely ruled out the possibility of radioactive releases.

The NRC said studies on a limited number of nuclear power plants by federal research labs and agency staff showed that even if there were initial releases of radioactivity (search), plant operators would have time to take actions to reduce the impact on public health. It was the most expansive public comment to date on what might happen in a terrorist attack.

NRC Chairman Nils Diaz, in an interview with The Associated Press, said Monday that while "it is possible there would be some damage and there could be some (radiation) releases ... it is not probable."

Nevertheless, added Diaz, "We cannot rule out the possibility that damage would occur and radioactive releases would take place. We're saying it would be very difficult for significant damage to take place (and) to get a major release of radioactivity in a very short time."

The government and nuclear industry have been particularly concerned since the Sept. 11, 2001, terrorist attacks that Al Qaeda (search) might target a commercial nuclear power plant. There is some evidence that a reactor may have been a potential target when the 2001 attacks were being planned.

Before 2001, neither the nuclear industry nor its government regulators had seriously considered the vulnerabilities of a reactor to a deliberate crash of a large aircraft loaded with fuel.

Since then, the NRC has been examining a number of classified engineering studies on such an attack. It has been using research from the Los Alamos (search) and Sandia (search) national laboratories as well as its own studies to determine how vulnerable commercial power reactors are to such an attack.

In the facilities analyzed, the studies found the likelihood of damaging the reactor core and releasing radioactive material that could affect public health and safety is low, Diaz wrote in a Sept. 8 letter to Homeland Security Secretary Tom Ridge.

Diaz wrote that "in the unlikely event" that a crashing aircraft would cause a radiation release, "there would be time to implement the required on-site mitigating actions" to protect public health.

Elaborating on the letter, Diaz said Monday it is the agency's view that even if there is damage to key areas of the power plant, the extent of damage would not be so severe that actions cannot be taken to reduce the threat of significant radiation exposure to the public.

Nevertheless, the NRC assessment appeared less certain that an industry-backed study released in late 2002, which said categorically that a large jetliner would fail to penetrate a nuclear power plant's concrete containment dome.

That study, conducted by the Electric Power Research Institute (search), concluded that engineering models showed that a fully fueled Boeing 767 would fail to breach a reactor's four-foot-thick concrete containment dome. The industry cited the study as showing there would be no radiation release.

Marvin Fertel, vice president of the Nuclear Energy Institute (search), the industry trade group, said he saw no conflict between the industry-backed study and the NRC findings. He said the government studies, details of which are classified, "apparently looked at other parts of the plant and reached basically the same conclusion we did that it's very hard to get a large release."

Diaz said the NRC conclusions were based on data that involved more than just the impact of an aircraft on the reactor containment dome. He said more than one containment dome design was studied as well as the potential impact of an aircraft on different parts of a power plant complex where damage might have an effect on plant safety and operation.